Nothing. Viennas subway has semi autonomous trains since 1960 or so. The driver just presses a “leave station” button after verifying nobody is stuck in the doors.
Stockholm metro has this as well but I wouldn't call it "autonomous", it's just a departure button to get the train moving faster. The driver still has to grip the vigilance device after a certain amount of time.
In Vienna it's fully automated between stations. The main reason it's not fully automated is because there are no automatically closing doors in the stations. U5 is going to get it though: https://www.youtube.com/watch?v=VBDqq7rwLE0
You can quite often see the trains operate fully automatic though when the drivers are changing shifts at the end of the line. The driver typically leaves and locks the train, lets it reverse itself and then the next driver continues: https://www.youtube.com/watch?v=0eyjqS9lvNA
